Guglielmo Marin

Male competition and female choice in Padogobius martensi (Pisces, Gobiidae)

Abstract In natural populations of Padogobius martensi, a small freshwater goby, the number of eggs per nest increases with the size of the guarding male. The present study is a laboratory investigation of factors determining this size-related reproductive success of males. Females mated randomly with respect to male size but showed a marked preference for spawning in large nest sites, which were always occupied by the larger males. Courtship interference, nest take-over and egg cannibalism reduced the reproductive success of smaller males, although the progeny of some small males were guarded by the larger males that took over their nest.

Sexual selection and sexual size dimorphism in the eastern mosquitofish Gambusia holbrooki (Pisces Poeciliidae)

We have previously shown (BISAZZA & MARIN 1991) that female eastern mosquitofish, Gambusia holbrooki, did not discriminate among males of different size, although they showed a preference for the dominant male in a group of males. In this study we examined the relationship between body size and the success of gonopodial thrusting, a mating tactic characterisitc of the poeciliids that enables a male to achieve copulation without the females cooperation. Without competitors the probability that a mating attempt was successful decreased exponentially with male length while it increased with the size of the female. When tested in a group, males competed for access to females. The largest individual monopolized access to females and accomplished the majority of mating attempts, whatever the number of its competitors. We used these results and demographic data (length distributions and composition of groups) of a natural population to simulate how the different mechanisms of sexual selection may interact under...

LONG LIVING AND REPRODUCTION SKIPPING IN THE FAT DORMOUSE

We studied the reproductive strategy of a population of fat dormice (Myoxus glis) breeding in nest boxes in a beech forest on the southern Alps, Italy. In eight years of data collection (1991-1998), we observed births only in 1991, 1992, and 1995. We did not observe young during the autumn dispersal in the remaining five years. Reproduction was not correlated with high rainfall, low temperature in summer, or with population density. Reproduction was only observed in years of beech mast seeding. Body condition of adults when they emerged from hibernation did not differ between years with and without repro- duction, suggesting that the adults may avoid breeding when the young have a low prob- ability of surviving their first winter. A capture-recapture analysis on individually marked adult fat dormice identified a very high yearly survival rate (0.86-0.92), close to that of large herbivorous mammals and much higher than that typical of other rodent species with similar body size. No influence of sex on survival or recapture probability was detected. The long life span expectancy of an adult dormouse (averaging nine years) is the prerequisite for the infrequent breeding strategy adopted by this rodent species to cope with an unpre- dictable food source. A high survival rate despite the small body size may have been possible because of the long hibernation time ( .6 months). Long life span and intermittent breeding make the life history of fat dormice unique among rodents.

Male Size and Female Mate Choice in the Eastern Mosquitofish (Gambusia holbrooki: Poeciliidae)

WEYGOLDT, P. 1980. Complex brood care and reproductive behavior in captive poison-arrow frogs, Dendrobates pumilio 0. Schmidt. Behav. Ecol. Sociobiol. 7:329-332. WOLDA, H. 1983. Spatial and temporal variation in abundance in tropical animals, p. 93-105. In: Tropical rain forest: ecology and management. S. L. Sutton, T. C. Whitmore and A. C. Chadwick (eds.). Blackwell Scientific Publications, Oxford, England.

Genetic variation and bill size dimorphism in a passerine bird, the reed bunting Emberiza schoeniclus

In passerine birds morphological differentiation in bill size within species is not commonly observed. Bill size is usually associated with a trophic niche, and strong differences in it may reflect the process of genetic differentiation and, possibly, speciation. We used both mitochondrial DNA (mtDNA) and nuclear microsatellites to study genetic variation between two subspecies of reed bunting, Emberiza schoeniclus schoeniclus and E.s. intermedia, along their distributional boundary in western Europe. These two subspecies are characterized by a high dimorphism in bill size and, although breeding populations of the two subspecies are found very close to each other in northern Italy, apparently no interbreeding occurs. The observed morphological pattern between the two subspecies may be maintained by geographically varying selective forces or, alternatively, may be the result of a long geographical separation followed by a secondary contact. MtDNA sequences of cytochrome b and ND5 (515 bp) showed little variation and did not discriminate between the two subspecies, indicating a divergence time of less than 500 000 years. The analysis of four microsatellite loci suggested a clear, although weak, degree of genetic differentiation in the large‐ and small‐billed populations, as indicated by FST and RST values and genetic distances. The correlation between bill size and genetic distance between populations remained significant after accounting for the geographical distances between sampling localities. Altogether, these results indicate a very recent genetic differentiation between the two bill morphs and suggest that a strong selection for large bills in the southern part of the breeding range is probably involved in maintaining the geographical differentiation of this species.

Induction of multipolar spindles by X-radiation in mammalian cells in vitro

Coverslip cultures of mammalian cells in logarithmic growth were exposed to 200, 400, and 600 r x radiation and fixed at 12-hr intervals up to 108 hr after exposure. The generation time of untreated cells was about 24 hr. Mitotic figures were scored and classed according to their number of poles at different intervals after irradiation. The relative number of mitoses with a high number of poles increased steadily with time, suggesting that multipolarity was a secondary consequence of radioinduced blockage of division rather than a primary effect of x radiation on duplication of the centrioles. The shift in the relative proportion of high and low multipolar spindles was more pronounced at the highest dose, where division hindrance was more complete. (C.H.)

Variations in male body size in natural populations of Gambusia holbrooki

Seven populations of Gambusia holbrooki in seven different locations, showing different physical characteristics, were sampled periodically for over 1 year. Population density differed across locations and seasons. It varied widely during the reproductive season, following a similar pattern in all locations. In all populations the sex-ratio tended to became more male biased during the breeding season. Average male size at maturity also showed significant differences in place and time: it always tended to be smaller when population density was low and the sex-ratio was female-biased. This trend is in agreement with previous laboratory experiments which had shown a higher mating efficiency for small males when intrasexual competition was absent.

Determinants of size in male eastern mosquitofish (Gambusia holbrooki): Inheritance and plasticity of a sexual selected character

Abstract We studied the heritability of size at maturity in male mosquitofish, Gambusia holbrooki, by setting up independent crosses between individuals derived from a wild population of northeastern Italy. Pairs included average‐sized females and males encompassing the range of sizes observed in nature. Progenies were split and siblings were grown either individually or in groups, under otherwise identical conditions. The heritability, calculated from the regression of the mean length of male offspring against the length of their fathers, was very low in both conditions. Length at maturity was significantly greater for males raised in groups. Under this condition, some males delayed maturation and attained a larger size. We also tested the effect of different photoperiods (9 h of light and 15 h of dark, or viceversa) on size at maturity. Short photoperiods caused males to mature earlier, at a smaller size. As in other poeciliids, the trait size at maturity showed considerable phenotypic plasticity, possi...

Winter fattening strategies of two passerine species: environmental and social influences

We studied the fattening strategies of two insectivorous passerines, the Robin Erithacus rubecula and the Wren Troglodytes troglodytes, during their wintering stage in a study area of the eastern Po valley (Northern Italy). We analysed the relationship between some environmental (temperature, photoperiod, rainfall) and social factors (density of migrant competitors, territoriality, home range size) on the one hand, and levels of fat reserve on the other. Although average body fat depots differed between the two species, both increased their daily fat stores as the photoperiod shortened with the progress of winter. The amount of fat accumulated by day was more than that required for overnight consumption. Both species also responded to short-term variations in weather conditions: fat depots were inversely related to the maximum temperature on the day of capture. The use of space and the social structure of the wintering populations seem to affect fattening strategies: fat reserves of resident Robins were negatively related to the density of conspecific migrants, early in the season. The same relationship was not observed for Wrens, which were dispersed in extensively overlapping home-ranges. In this species, the home-range size was positively, and fat reserves negatively, correlated with the body size of the bird.

No evidence of extra‐pair paternity in a colonial seabird, the common tern (Sterna hirundo)

Abstract The incidence of extra‐pair paternity and egg dumping was investigated in a colony of common terns (Sterna hirundo), a colonial seabird, in the Venetian lagoon. Ten families were sampled and multilocus DNA fingerprinting analysis was performed. No indication of extra‐pair paternity or egg dumping was found in any of the families. The results are discussed in the light of life‐history strategies, the benefits of coloniality and the evolution of adoption behaviour in the species.